This application is based on application No. 2001-332937 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a tube structure of a metal halide lamp having a ceramic arc tube made of such material as alumina ceramic.
2. Related Art
A metal halide lamp is a high intensity discharge lamp in which a metal halide is enclosed in its arc tube. For conventional metal halide lamps, silica is used as a material for the arc tube. Recently, however, ceramic is getting attentions to replace the silica, since the silica-made arc tube has not yielded enough strength against the high temperature and high pressure resulting from the light emission.
Specifically, the ceramic material has 1,200xc2x0 C. heat resistance, as opposed to that of the silica material which is 1,000xc2x0 C. The ceramic material also has better mechanical strength compared to the silica material, which has led to a vigorous development attempts for an arc tube that uses ceramic materials.
However, in order to realize a metal halide lamp with a high luminous efficacy and a better color-rendering characteristic, it is necessary to apply a greater power to the metal halide lamp. In such a case, an arrangement such as downsizing the arc tube becomes necessary in order to raise the temperature in an attempt to increase the vapor pressure of a metal halide. This increases a possibility of the arc tube being broken even using ceramic material for the arc tube.
Tests have been conducted as for the quality of the commercially available metal halide lamp made of ceramic. The results indicate that (1) most breakable type of metal halide lamp is one which has a great power consumption, in particular 70 W or more, (2) occurrence of break is more frequent for higher voltage of the power source than the normal power rating, and (3) a larger number of metal halide lamps are broken by longer aging time.
Specific data for (3) shows that only a few percent of the total metal halide lamps examined are broken before the aging hours exceed 6,000 hours. Note that this tendency of quality applies for all types of arc-tube structure, whether it is an integrally-molded arc tube, or an arc tube in which a plurality of members have been fit by shrink fit.
The object of the present invention, in view of the above-described problems, is to provide a metal halide lamp equipped with a ceramic arc tube, which has a high luminous efficacy and a color-rendering characteristic, and is hard to break.
In order to achieve the above object, a metal halide lamp relating to the present invention is characterized by including a ceramic arc tube vessel which has a central tube member and two thin tube members, the central tube member having a larger outside diameter in a middle part than in two ends in a longitudinal direction, and one of the thin tube members being inserted to one of the two ends, and the other thin tube member being inserted to the other end, where the central tube member, on an external surface, has two transitional areas which each have a minimum curvature radius of 0.3 mm or more, each transitional area being between the middle part and a corresponding one of the ends. Here, a power consumption of the metal halide lamp may be in a range of 70 W to 400 W inclusive.
The above construction prevents microcracks that would occur at the ends of the central tube member due to the lamp aging lighting, which is especially effective for a high-wattage metal halide lamps, which uses an arc tube vessel which includes the ceramic central tube member and two ceramic thin tube members. According to the construction, a break at such ends is assuredly prevented. This realizes a metal halide lamp made of alumina ceramic which is high-wattage and is of high quality in terms of lifespan as a lamp.
Note that the microcrack here means a crack of about {fraction (1/1000)} mm that is impossible to be observed by the naked eyes.
Further, the arc tube vessel may be made of alumina ceramic.
According to the above construction, light dispersion through the alumina particles will attain a good light distribution characteristic.
In addition, a metal halide lamp relating to the present invention is characterized by including an arc tube vessel made of ceramic, the arc tube vessel having a larger outside diameter in a middle part than in two ends in a longitudinal direction, where the arc tube vessel, on an external surface, has two transitional areas which each have a minimum curvature radius of 0.3 mm or more, each transitional area being between the middle part and a corresponding one of the ends. For this type of metal halide lamp, the maximum value for the minimum curvature radius at each of the transitional areas is preferably 7 mm. According to the above construction, the object of the present invention is achieved on the metal halide lamp including an arc tube vessel that is integrally molded. Here, a power consumption of the metal halide lamp may be in a range of 70 W to 400 W inclusive.
Here, the integrally molded arc tube vessel may be also made of alumina ceramic. This construction enables to realize a good distribution characteristic for an integrally molded arc tube vessel.
In addition, a more concrete form of a metal halide lamp relating to the present invention may be a metal halide lamp including a ceramic arc tube vessel that has a central tube member and two thin tube members, one of the thin tube members being inserted to one of two ends of the central tube member, and the other thin tube member being inserted to the other end, where the central tube member is made by integrally-molding thereon the two ends thereof which each have a cylindrical-shape, a central part which has a larger diameter than a diameter of the two ends, and two taper parts which are each shaped substantially as a truncated cone, one of the taper parts connecting one of the two ends to a corresponding one of two end portions of the central part, and the other taper part connecting the other end to the other end portion of the central part, and the central tube member, on an external surface, has two transitional areas which each have a minimum curvature radius of 0.3 mm or more, each transitional area being between one of the ends of the central tube member and a corresponding taper part.
In addition, a metal halide lamp that has an integrally molded arc tube vessel may be realized by a metal halide lamp including an arc tube vessel made of ceramic, the arc tube vessel is formed by integrally-molding two thin tube parts which each are in a cylindrical-shape, a central part which has a larger diameter than the thin tube parts, and two taper parts which are each shaped substantially as a truncated cone, one of the taper parts connecting one of the thin tube parts with one of two ends of the central part, and the other taper part connecting the other thin tube part with the other end of the central part, where the arc tube vessel, on an external surface, has two transitional areas each have a minimum curvature radius of 0.3 mm or more, each transitional area being between one of the thin tube parts and a corresponding taper part.
According to the above construction enables the effect that the present invention tries to achieve.